Chromium speciation analysis by separation of Cr(III) from Cr(VI) on a XAD sorbent derivatized with shellac:: a natural polymer

A XAD-shellac sorbent, synthesized by the impregnation of the natural polymer shellac ( purified product of the hardened resinous secretion of the lac insect Kerria lacca) on Amberlite XAD-16 copolymer backbone, has been developed for the separation of Cr(III) from Cr( VI), and preconcentration of Cr( III) from synthetic solutions and real samples. The preconcentration factor for Cr( III) was 75. All chromium determinations were made using the diphenyl carbazide spectrophotometric method after oxidizing Cr to chromate( VI) where necessary, and simultaneously with flame-AAS for confirmation. The dynamic breakthrough and batch capacities of this sorbent for Cr( III) were 0.3 and 0.9 mg g(-1), respectively, indicating that the ion-exchange mechanism was prevalent in the dynamic mode, whereas in the batch mode, the surface sites were also capable of exerting their chelating effects. When XAD-shellac was thoroughly washed with ammonium acetate solution prior to use in chromium speciation, the cationic (RH+) surface sites were probably neutralized to yield free acetic acid, and the resulting resin did not retain CrO42-. Thus, complete separation and speciation of Cr( III) from CrO42- was possible using this sorbent. The shellac-coated sorbent decomposed in alkaline solution (i.e. over pH 7.5), and therefore the retained Cr( III) was eluted with dilute (0.025 - 0.050 M) HCl. Thus, Cr( III) in admixture with Cr( VI) could be separated and recovered, without interference from the hexavalent state. XAD-shellac was not successful for Cr preconcentration from seawater, but was efficiently used for synthetic and real electroplating wastewater and CRMs such as SO-2 soil, San Joaquin soil, BCR 145R sewage sludge, with a recovery ratio for Cr( III)/Cr(VI) extending up to <= 98%.